This invention relates to a heat transfer sheet which uses a sublimable dye (heat migratable dye), more particularly, to a heat transfer sheet which has solved problems of printing wrinkle and image dislocation due to slippage generated during heat transfer printing.
Various heat transfer methods have been known in the art, and among them, there has been proposed a method in which a sublimable dye is used as the recording material. The dye is carried on a substrate sheet such as a polyester film to form a heat transfer sheet. By using the transfer sheet, various full colors are formed on an image-receiving sheet having a dye receptive layer with a sublimable dye formed on a substrate such as paper or plastic film. In this case, a thermal head of a printer is used as the heating means, and a large number of color dots of 3 or 4 colors are transferred onto the image-receiving sheet by heating for a very short time, thereby reproducing the full-color image of the original with the multi-colored color dots.
The thus formed image is very sharp, since the colorant used is a dye and also excellent in transparency, whereby the obtained image is excellent in reproducibility and gradation of the intermediate color, similar to the image according to the offset printing and gravure printing of the prior art, and further can form an image of high quality comparable with full-color photographic images.
As the substrate film of the above heat transfer sheet, papers such as condenser paper may be sometimes employed, but such thin paper is lower in strength, particularly weak in bursting strength and therefore, it is desirable to use a film having a tough plastic nature such as polyester resin as the substrate film.
However, in this case, the following problems will further ensue. That is, the transfer sheet is thermally deformed due to the heat of a temperature of 250.degree. to 300.degree. C. or higher being locally loaded from the thermal head to the heat transfer sheet during printing. Further, the heat transfer sheet is conveyed under pressurization of a thermal head and is nonuniformly elongated, whereby a large number of wrinkles are generated on the sheet. As a result, not only running under the thermal head is obstructed, but also slippage and drop-out of the dots are generated in the obtained image, thus involving the problem that the resolution of the printed image is lowered, and also that color reproducibility is lowered in formation of full-color. Such problems become particularly conspicuous when a marked density difference is needed for the image to be formed, because the heat content imparted to the thermal head has a locally great difference.
The problems as mentioned above can be alleviated by use of a substrate having a relatively greater thickness, but in this case, sensitivity of the heat transfer sheet is lowered to become practically useless.
In another method, it has been proposed to provide a heat-resistant protective layer such as a thermosetting resin on the surface opposite to the dye layer. However, even by use of these methods, if the heat-resistant protective layer is made thick to the extent effective for prevention of printing wrinkle phenomenon, sensitivity of the heat transfer sheet and resolution of the printed image are lowered, and therefore they cannot be satisfactory measures of solution.
Accordingly, an object of the present invention is to provide a heat transfer sheet capable of giving the image which is excellent in sharpness and resolution and has sufficient printing density without causing printing wrinkles and image slippage.